The Retinex theory on a model of human visual perception conventionally used in performing a gradation correction processing and the like, is disclosed in E H Land and J J McCann, “lightness and retinex theory,” J. Opt. Soc. Am. 61, 1 (1971). Based on the retinex theory, an image is separated into an illumination component and a reflectance component and a gradation correction processing is performed only on the illumination component (see, for example, Kuang, J., Johnson, G. M., Fairchild M. D. “iCAM06: A refined image appearance model for HDR image rendering,” Journal of Visual Communication and Image Representation, Volume 18, Issue 5, October 2007, Pages 406-414). In this way, a visually good processed image is obtained.
Here, the gradation correction processing is a conversion for narrowing a dynamic range of an image or adjusting the brightness of a dark section and a bright section. The illumination component is a component image largely influenced by illumination light and the reflectance component is a component image largely influenced by the reflectance of a subject. Further, in performing a texture correction processing, a visually good processed image is obtained if the correction processing is performed only on the separated reflectance component. These correction processings may be simultaneously performed on the respective components of one image.
If the illumination component is not appropriately obtained particularly in an area adjacent to an edge in generating an illumination component image described in the Retinex theory, a halo artifact appears in an area adjacent to an edge of an image after the gradation correction. To solve this problem, it is necessary to use a nonlinear filter (e.g. bilateral filter) for performing smoothing in areas other than edges while preserving the edges such as the outline of an object and widen a reference pixel range of the filter. Thus, a huge amount of computation is necessary to generate the illumination component image.
Accordingly, a fast computational technique such as fast bilateral filtering has been, for example, proposed. The fast bilateral filtering is disclosed in the following literature: “Fast Bilateral Filtering for the Display of High-Dynamic-Range Images” written by Fredo Durand and Julie Dorsey, SIGGRAPH 2002.